Alternative substrates for epitaxial growth

ABSTRACT

A substrate including a base substrate, an interfacial bonding layer disposed on the base substrate, and a thin film adaptive crystalline layer disposed on the interfacial bonding layer. The interfacial bonding layer is solid at room temperature, and is in liquid-like form when heated to a temperature above room temperature. The interfacial bonding layer may be heated during epitaxial growth of a target material system grown on the thin film layer to provide the thin film layer with lattice flexibility to adapt to the different lattice constant of the target material system. Alternatively, the thin film layer is originally a strained layer having a strained lattice constant different from that of the target material system but with a relaxed lattice constant very close to that of the target material system, which lattice constant is relaxed to its relaxed value by heating the interfacial bonding layer after the thin film layer is removed from the first semiconductor substrate, so that the thin film layer has an adjusted lattice constant equal to its unstrained, relaxed value and very close to the lattice constant of the target material system.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/820,072, filed Mar. 28, 2001, which claims, under 37 C.F.R.§ 1.78(a)(3), the benefit of the filing date of provisional U.S.national application No. 60/208,115, entitled “Fabrication of VerticalCavity Surface Emitting Lasers Using Alternative Substrates,” by Wen-YenHwang, filed May 31, 2000 under 35 U.S.C. § 111(b), which are bothincorporated herein in their entireties by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention is related to substrates for epilayer epitaxialgrowth in which the epilayers are lattice mismatched to the substrateand, in particular, to alternative substrates for fabrication ofelectronic and optoelectronic devices, such as semiconductor diodelasers, for example vertical-cavity surface-emitting lasers (VCSELs).

[0004] 2. Description of the Related Art

[0005] The following descriptions and examples are not admitted to beprior art by virtue of their inclusion within this section.

[0006] Lasers, such as semiconductor diode lasers, have a wide range ofindustrial and scientific uses. The use of semiconductor diode lasers assources of optical energy is attractive for a number of reasons. Forexample, diode lasers have a relatively small volume and consume a smallamount of power as compared to conventional laser devices. Further, asmonolithic devices, they do not require a combination of a resonantcavity with external mirrors and other structures to generate a coherentoutput laser beam. One disadvantage of the semiconductor diode laser,however, is the relatively low power of the output beam, as compared toother types of laser devices.

[0007] Group III-V (“III-V”) semiconductor materials have been used toconstruct semiconductor lasers. Processing of III-V semiconductordevices includes vital steps for depositing III-V materials on asemiconductor substrate. For the deposition of a thick III-V layer, thelattice constant of the substrate material has to be very close to thatof the deposited III-V layers (epi layers) with the same crystallinestructure. Otherwise, crystalline defects, especially threadingdislocations, will form during material deposition. When the defectdensity in the deposited material is high, it will significantly degradedevice performance. These threading dislocation defects can createleakage paths for current, provide undesired carrier recombinationcenters and reduce device lifetime.

[0008] It is thus very difficult to grow high quality thin filmmaterials on conventional prior art substrates with a large latticemismatch. This lattice-matching requirement for compound semiconductormaterial deposition severely limits the possible choice of compoundsemiconductor material compositions and device material structuredesigns due to the limited choice of available substrates with theappropriate crystalline structures and lattice constants. Suchsubstrates include Si, GaAs, InP, GaSb, InAs, and sapphire, inter alia.

[0009] For material systems for which there are no lattice-matched priorart substrates, however, some alternative approaches have been used.E.g., either a thick buffer layer is grown on the substrate, as proposedin U.S. Pat. No. 5,285,086, or a special technique, such as the lateralgrowth method proposed by Parillaud et al., Appl. Phys. Lett. vol. 68(1996), p. 2654, is employed before the growth of the device structurelayers. It is known that defects, in particular threading dislocations,induced by lattice mismatch can be reduced from b 10 ¹¹/cm² to 10⁵/cm²by using the lateral growth method, for example. However,lattice-mismatched material growth techniques that result in defects,especially threading dislocation defects, often cause undesirableperformance or characteristics of optoelectronic or electronic devicesgrown with such techniques.

[0010] It is desirable to epitaxially fabricate a variety of types ofstructures or devices, using a given epi material system, grown on agiven substrate. Such epitaxially fabricated devices include electronicdevices, such as transistors and integrated circuits, and optoelectronicdevices, such as semiconductor lasers, light-emitting diodes, andphotodetectors.

[0011] One such optoelectronic device in which there has recently beenan increased interest is the vertical-cavity surface-emitting laser(VCSEL). The conventional VCSEL has several advantages, such as emittinglight perpendicular to the surface of the die, and the possibility offabrication of two dimensional arrays. VCSELs typically have a circularlaser beam and a smaller divergence angle, and are therefore moreattractive than edge-emitting lasers in some applications. Longinfra-red spectrum wavelength (e.g., the range from approximately 1.2 μmto approximately 1.8 μm, including closely-spaced wavelengths around 1.3μm or closely-spaced ITU grid wavelengths around 1.55 μm) VCSELs arealso of great interest in the optical telecommunications industrybecause of the minimum fiber dispersion at 1.32 μm and the minimum fiberloss at 1.55 μm. The dispersion shifted fiber will have both minimumdispersion and minimum loss at 1.55 μm. The long wavelength VCSEL istypically based on an In_(x)Ga_(1−x)As_(y)P_(1−y) active layer latticematched to InP cladding layers.

[0012] The structure of a typical VCSEL usually consists of an activeregion sandwiched between two distributed Bragg reflector (DBR) mirrors,as shown schematically in FIG. 1. For the fabrication of long wavelength(e.g., 1.3 or 1.55 μm) VCSELs, it is very difficult to form the desiredmaterials in one single growth step on a substrate. For instance, it isdifficult to grow either the desired 1.3 μm active region on a GaAssubstrate or to grow proper DBR mirrors on an InP substrate, despite thematurity of the technology for growing the DBR structure on GaAssubstrates. Likewise, it is difficult to grow a 1.3 μm wavelength DBRstructure on an InP substrate, despite the maturity of the technologyfor growing the active region. Recently, some alternative materialsystems, such as InGaNAs, GaAsSb and InGaAs quantum dots, have beendeveloped to grow directly on a GaAs substrate using anAl_(x)Ga_(1−x)As/Al_(y)Ga_(1−y)As DBR for a 1.3 μm wavelength activeregion. However, these material systems are very difficult to grow andnot easy to reproduce.

[0013] Another alternative approach to fabricate a long wavelength VCSELis by using the so-called wafer bonding technique. However, thisapproach requires at least two to three wafer growth and one to twowafer-to-wafer bonding processes, which leads to very high fabricationcost and very low device yield. Therefore, a single wafer growthapproach would be preferable to the wafer bonding approach, otherconsiderations being equal.

[0014] One alternative approach to fabricate a long wavelength VCSELwith a single wafer growth step is to use the (In,Ga,Al)As materialsystem lattice matched to In_(x)(Al_(y)Ga_(1−y))_(1−x)As, where, e.g.,0.15<×<0.45, and growth of an InAlGaAs/InAlAs DBR structure and amoderately strained InGaAs quantum well (QW) structure active region.(Depending on the value of x, y is selected such that the materialutilized has a bandgap absorption edge less than the lasing wavelength,e.g. less than 1.3 μm for a 1.3 μm VCSEL.) However, there is nocommercially available substrate that is lattice matched to thismaterial system. It is very difficult to control the compositionprecisely of a ternary In_(x)Ga_(1−x)As substrate uniformly over a wholewafer. Therefore, a high quality alternative substrate needs to bedeveloped for this application.

[0015] One approach is to create a substrate that has the samecrystalline structure and the same surface lattice constant as those ofnon-strained In_(x)(Al_(y)Ga_(1−y))_(1−x)As, where 0.15<×<0.45. Anotherapproach is to make a substrate that has a thin layer that is physicallyattached to the substrate, but can freely expand in a direction parallelto the substrate surface during material growth. This thin surface layermust have the same crystalline structure and a similar lattice constantas those of non-strained In_(x)(Al_(y)Ga_(1−y))_(1−x)As, where0.15<×<0.45.

[0016] For lattice-mismatched epitaxial layers, it is widely acceptedthat there exists a critical thickness beyond which misfit dislocationsare introduced causing the breakdown of coherence between the substrateand epitaxial layers. The relaxation mechanism for lattice-mismatchedepilayers known as the Matthews-Blakeslee model, and other aspects ofepitaxial layer lattice mismatching problems are discussed in J. W.Matthews, S. Mader & T. B. Light, J. Appl. Phys. 41 (1970): 3800; J. W.Matthews & A. E. Blakeslee, “Defects in Epitaxial Multilayers I,” J.Qryst. Growth 27 (1974): 118-125; J. W. Matthews & A. E. Blakeslee,“Defects in Epitaxial Multilayers II,” J. Cryst. Growth 29 (1975):273-280; J. W. Matthews & A. E. Blakeslee, “Defects in EpitaxialMultilayers III,” J. Cryst. Growth 32 (1976): 265-273; and J. W.Matthews, J. Vac. Sci. Technol. 12 (1975): 126.

[0017] U.S. Pat. No. 5,294,808 for “Pseudomorphic and Dislocation FreeHeteroepitaxial Structures” proposes to use a thin substrate having athickness on the order of the “critical” thickness, which is thethickness at which defects form when growing one lattice mismatchedmaterial on another. The critical thickness is only a few hundredangstroms, and it is difficult to sustain the mechanical and chemicalprocesses required for epitaxial (epi) growth and device fabrication ona substrate having a thickness of only a few hundred angstroms. However,in practical situations, after the thin substrate is bonded to thesupporting substrate, the bonding strength between the interface is sostrong that this thin substrate can no longer freely change its latticeconstant in the in-plane direction. Therefore, threading dislocationswill still be generated due to the very limited strain accommodation inthe thin substrate.

[0018] There is, therefore, a need for improved substrates andfabrication techniques that address the foregoing problems. In general,there is a need for alternative substrates that can be used for avariety of epi material systems without giving rise to conventionalproblems caused by lattice mismatch between the epi layers and thesubstrate. For example, there is a need for alternative substrates thataddress the problems associated with lattice mismatching between asubstrate and the (In,Al,Ga)As material intended to be used for long(e.g., 1.3 or 1.55 μm) wavelength VCSELs or other special materialsystems. Such alternative substrates could be advantageously used forother material systems and device structures as well, and in general forany material system for which other substrates cannot satisfy thelattice-matching requirement for device applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the accompanying drawings in which:

[0020]FIG. 1 is a cross-sectional view of a typical layer structure of avertical cavity surface emitting laser (VCSEL) device;

[0021]FIG. 2 is a schematic diagram illustrating a generic layeredstructure employed to form defect-free epitaxial layers on analternative substrate, in accordance with an embodiment of the presentinvention;

[0022]FIGS. 3A, B, C, D, and E are schematic illustrations of a processemployed to fabricate an alternative substrate in accordance with anembodiment of the present invention;

[0023]FIGS. 4A, B, C, and D are schematic illustrations of anotherprocess employed to fabricate an alternative substrate in accordancewith an embodiment of the present invention;

[0024]FIG. 5A is a schematic illustration of the epi-up configurationfor laser mounting in output power measurement, according to theinvention; and

[0025]FIG. 5B is a schematic illustration of the prior art epi-downconfiguration for laser mounting in output power measurement.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Commonly-owned U.S. patent application Ser. No. 09/426,273, filedOct. 25, 1999, for “Compliant Universal Substrates for Optoelectronicand Electronic Devices,” now issued as U.S. Pat. No. 6,406,795, isincorporated by reference herein in its entirety.

[0027] To address the problem of lattice mismatch between a prior artsubstrate and (In,Al,Ga)As material intended to be used for a longwavelength (e.g., 1.3 or 1.55 μm) VCSEL or other special materialsystems, an alternative substrate is provided for growth of high-qualitypseudomorphic epitaxial (epi) thin films without generating high-densitythreading dislocations. The alternative substrate of the presentinvention preferably allows high quality compound semiconductor thinfilm growth and also endures all the material epitaxy and devicefabrication processing steps. The present invention provides alternativesubstrates using wafer fusion or wafer bonding techniques thatfacilitate the formation of high quality devices on these alternativesubstrates.

[0028] According to the present invention, an alternative substrate hasa base layer and a thin film layer physically bonded to the substrate.Two basic approaches to providing an alternative substrate fordefect-free (or reduced defect) epitaxial, growth are disclosed herein:the floating substrate approach and the relaxed substrate approach,which are described in further detail below. Depending on the approachemployed, the adaptive thin film layer either (a) has a lattice constantdifferent from that of the target epi material system, but with asufficient degree of lattice flexibility during epitaxial growth of thetarget material system, due to the presence of a floating interfacialbonding layer, to permit the lattice constant of the adaptive thin filmlayer to adjust to that of the target system, thereby providing latticematch and reducing lattice mismatch threading dislocations; or (b) thethin film layer, which is initially strained and has lattice mismatchwith the target material system, has its in-surface lattice constantadjusted by relaxation before epi growth so that it has a latticeconstant very close to that of the target material system. In the secondapproach, the thin film layer may be bonded to a base layer with orwithout an interfacial bonding layer, in alternative embodiments.

[0029] The present invention thus provides an alternative substrate forthe formation of various devices with special target epi materialsystems, such as approximately 1.2 μm to approximately 1.8 μm wavelengthVCSELs. The alternative substrate includes a base layer and a thin filmcrystalline layer on and bonded to the base layer, with or without aninterfacial bonding layer, depending on the embodiment. The thin filmlayer's lattice constant is adjusted either during epi growth toaccommodate the different lattice constant of the target epi layers; oris adjusted prior to the epi growth to create a thin film layerlattice-matched to the target material system. In either case, aninterfacial bonding layer is employed to adjust the thin film layer'slattice constant, whether before or during epi growth. This approach canalso be used for other material systems for different deviceapplications, as will be appreciated by those skilled in the art.

[0030]FIG. 2 schematically illustrates an alternative substrate 20 inaccordance with an embodiment of the present invention. Alternativesubstrate 20 is a multilayer structure having a thick bulk material baselayer or substrate 21 and a thin film adaptive layer 22, which is bondedto the bulk material base layer 21 with an interfacial bonding layer 23.The thin film adaptive layer 22 serves as the actual substrate thatsupports growth of an epi layer 24 in a growth chamber. The thin filmadaptive layer 22 could have a thickness from approximately 5 μm or lessto approximately a few microns (μm), depending on application anddesign. The thin film adaptive layer 22 is mechanically robust when itis bonded to the base layer 21. The interfacial bonding layer 23 may bea thin metal(s) layer, an inorganic layer, or a combination of anymaterials listed below. Alternatively, interfacial bonding layer 23 maysimply be the interface formed between the treated (i.e., cleaned)surface layers substrate 21 and thin adaptive layer 22, in alternativeembodiments.

[0031] When bonding layer 23 is a metal layer, for example, it may beformed of one or more metal layers. For example, it may be asingle-layer of Bizmuth (Bi), or other metals such as Pb, In, Sn, Sb,Al, or the like. Some alloys with low melting temperatures (e.g., <600°C. or 500° C.) can also be used, such as In:Sn, Pb:Sn, In:Pb, In:Ag, orother element or alloy. Bonding layer 23 may have a single layer, ortwo, three, or more layers of metals and/or alloys. As described below,the layer structure and materials of the metal bonding layer 23 arepreferably selected to avoid undesired chemical reaction with the thinadaptive layer 22 and supporting substrate 21.

[0032] The layer 23 is preferably in solid form at temperatures under100° C. and provides sufficient bonding strength between the layers 21and 22 to hold them together for material processing and fabricationpurposes, i.e., for mechanical stability. The interfacial bonding layer23 may actually comprise a thin interfacial layer that could vanishafter bonding layer 22 to layer 21, or could be as thick asapproximately a few micrometers, depending, in part, on the embodiment.

[0033] In accordance with an embodiment of the invention, there are twodifferent basic techniques to make an alternative substrate for VCSELs(e.g., for 1.3 or 1.55 μm wavelength light output) based onIn_(x)(Al_(y)Ga_(1−y))_(1−x)As, where, for example, 0.15<×<0.45. Otherranges for x may also be employed in alternative embodiments. It isunderstood that all mole fractions hereinafter are exactly orapproximately the value at the indicated extremes previously givenunless otherwise indicated, as will be appreciated by those skilled inthe art.

[0034] According to a first embodiment or approach, the thin adaptivelayer 22 is made to accommodate the difference between its latticeconstant and that of the epitaxial layer during epitaxial growth oflayer 24, which will be referred to as the “floating substrateapproach.” According to a second embodiment or approach, the in-planelattice constant of the thin adaptive layer 22 is modified beforeepitaxial growth, such that it has the same (in-plane) lattice constantas that of the target material during epitaxial growth of layer 24. ThisWill be referred to as the “relaxed substrate approach.” For the case ofa 1.3 or 1.55 μm VCSEL, the target material can be, for example,In_(x)(Al_(y)Ga_(1−y))_(1−x)As, where 0.15<×<0.45. Other target materialsystems may be employed as well.

[0035] Floating Substrate Approach

[0036] According to one embodiment, the floating substrate approach isachieved using the interfacial bonding layer 23 between the thinadaptive layer 22 and the supporting substrate 21, as indicated above.In this approach, thin adaptive layer 22 has a different latticeconstant than that of the target material system, but has a latticeflexibility during epi growth. During epi growth of epi layer 24, theinterfacial bonding layer 23 becomes liquid, partially liquid, orliquid-like (hereinafter referred to as liquid-like) after the substrate21 is heated up to a temperature higher than room temperature (i.e.,“liquidizes”), for example, higher than 100° C. When the interfaciallayer 23 becomes liquid-like, the thin adaptive layer 22 physicallyfloats freely on the interfacial layer 23 and has the degrees of freedomto expand or contract its lattice constant to adapt to the latticeconstant of the epitaxial layer being grown thereon. The liquid-likestate of the interfacial bonding layer 23, after substrate heating,allows the layer 22 to change its lattice constant without generating(or reduces the occurrence of) threading dislocations, and/or channelsany threading disclocations into the adaptive layer instead of upward,into the growing epi layer 24.

[0037] Thus, in this embodiment, interfacial bonding layer 23 must notbe merely an interface that is vanishingly small; it must be an actuallayer having a finite thickness sufficient to perform the floatingfunction to give thin adaptive layer 22 lattice flexibility during epigrowth of epi layer 24. The interfacial layer 23 is preferably thinenough that its surface tension will not destroy the thin adaptive layer22 when it is in the liquid-like state. The interfacial layer 23 alsopreferably has a viscosity such that the thin adaptive layer 22 and theepitaxial layer 24 will not fall off or shift (e.g., slide laterally)during material epitaxy or device processing. The interfacial layer 23further does not react or alloy (or minimally reacts or alloys)chemically with the thin film adaptive layer 22, which reacting couldotherwise destroy layer 22 (e.g., when it is heated to becomeliquid-like during epi growth of epi layer 24).

[0038] Substrate or base layer 21 can be formed from any commerciallyavailable high quality substrate material, such as Si, GaAs, InP, GaP,or the like. However, the thin film adaptive layer 22 can be made fromeither the same material or a variety of other materials as the baselayer 21. Thin adaptive layer 22 is preferably a semiconductor layer tofacilitate growth thereon of semiconductor based epi layers and devicesin epi layer 24.

[0039] FIGS. 3A-E illustrate a method for fabricating an alternative(sometimes referred to as a compliant universal (CU)) substrate such asillustrated in FIG. 2, in accordance with the floating substrateapproach, according to an embodiment of the present invention. In FIGS.3A and 3B, first and second wafers 300 and 301 are provided, each ofwhich is formed from a suitable bulk substrate material, for example,Si, GaAs, InP, GaSb, GaP, InAs, or the like. It should be emphasizedthat any suitable material may be employed as the substrate material,including both semiconductor and non-semiconductor materials. The wafer300 may undergo a process (such as e-beam evaporation, thermalevaporation, or sputtering) to deposit a thin top bonding layer 302 onthe substrate 300, as shown in FIG. 3A.

[0040] In one aspect of the invention, as shown in FIG. 3C, an etch-stoplayer 333 is formed on an alternative second wafer substrate 331 havingthe thin adaptive layer 332 disposed thereon. The etch stop layer 333may be AlGaAs, InGaP, InAlP, or the like. The thin adaptive film layer332 is formed on the etch stop layer 333 using any suitable conventionaltechnique, such as molecular beam epitaxy (MBE); liquid phase epitaxy(LPE); or a vapor phase epitaxy (VPE) process such as or metalorganicchemical vapor deposition (MOCVD, also known as MOVPE). A thin bondinglayer also could be formed on top of the thin adaptive layer 332.Alternatively, in another embodiment, thin film adaptive layer 303 isformed directly on the second wafer 301, as shown in FIG. 3B.

[0041] Next, as illustrated in FIG. 3D, the second wafer 301 is invertedrelative to the first wafer 300, and the thin film layer 303 is bondedto the top surface bonding layer 302 of the first wafer 300. The joiningof the two wafers 300 and 301 can be the result of Van der Waals forces,hydrogen bonding, covalent bonding, ionic bonding, or the like, or anyother mechanism, and results in the bonding layer 302, as shown in FIG.3E, becoming a finite thickness interfacial layer. In some embodiments,pressure is applied during the wafer bonding process. Depending on thedetailed process conditions and the bonding mechanisms, the appliedpressure can vary from approximately zero to over approximately 10MPascal or higher. Finally, as illustrated in FIG. 3E, the conventionalsubstrate second wafer 301 is removed by a selective etching (or liftoff) technique, as will be appreciated by those skilled in the art. Theetch stop layer 333 may be used to prevent removal of any of the thinfilm layer 332 if the wafer 331 is used. The exposed thin film layer 332(or 303) can be used as a CU substrate platform for epitaxial growth,while the wafer 300 now becomes the supporting bulk material base layer(effective substrate).

[0042] Alternatively, instead of depositing bonding layer 302 onsubstrate 300, the thin bonding layer 302 could be deposited on top ofthe surface of thin adaptive layer 303 or 332, for wafer bonding,depending on the process design. Or, bonding layer 302 may be depositedpartially on substrate 300, and partially on adaptive layer 303 or 332,so that after the step shown in FIG. 3D, a bonding layer is disposedbetween thin adaptive layer 303 and the bulk material 300, as shown inFIG. 3E.

[0043] Whether bonding layer 302 is deposited on substrate 300, or layer302/332, or partially on both, the bonding layer may be a single ormultilayer metal, as described above with reference to bonding layer 23.

[0044] Relaxed Substrate Approach

[0045] In the relaxed substrate approach of the present invention, thelattice constant of the thin adaptive layer 22 (FIG. 2) is modifiedbefore bonding it to the supporting substrate 21. Preferably, thelattice constant of thin adaptive layer 22 is modified to match thelattice constant of the material system of epi layer 24 which is to begrown thereon. This is done by selecting the material for adaptive layer22 so that its relaxed (i.e., unstrained, or “natural,” or “original”)lattice constant is very close to, or the same as, that of a targetmaterial system to be grown as epi layer 24. Then, thin adaptive layer22 is grown on a first substrate having a different material and thus adifferent lattice constant than the thin adaptive layer, giving rise toa strained thin adaptive layer having a strained lattice structuredifferent than the non-strained (relaxed) lattice constant. However, thethin adaptive layer 22 is thin enough so that, although it is strained,there are no threading dislocations.

[0046] A first surface of the thin film layer is then bonded to asurface of a second, supporting substrate (incidentally having a latticeconstant different from that of the thin film layer), with a flexibleinterfacial bonding layer. The first substrate is removed, and aninterfacial bonding layer is used, similar to the relaxation approachdescribed above with respect to the floating substrate approach, torelax the thin adaptive layer, to adjust its lattice constant to itsnon-strained, relaxed value. Thereafter, the now-non-strained thinadaptive layer is preferably mounted on another substrate or support,e.g. bulk material (with our without a second interfacial bondinglayer), and the first bonding layer and its supporting substrate isremoved from the thin adaptive layer. The thin adaptive layer 22,supported by a bulk material substrate, may then be utilized to grow anepi layer thereon lattice matched to the non-strained lattice constantof the thin adaptive layer.

[0047] This process results in an alternative substrate for theformation of semiconductor devices, which substrate has a crystallinebase layer and/or bulk material, and, on the base layer, a thin filmlayer having a lattice constant very close to that of the targetmaterial system. In this approach, interfacial bonding layer 23 may bemerely an interface between the base and the adjusted thin film layer,because it need not provide the floating function during epitaxialgrowth of epi layer 24 that it provides in the floating substrateapproach. Alternatively, interfacial bonding layer 23 may be a reallayer having finite thickness.

[0048] Referring now to FIG. 4, there is illustrated a method offabricating an alternative. substrate in accordance with the relaxedsubstrate approach of the present invention. In FIG. 4, the foregoingsteps are illustrated in further detail with respect to carrier(support) substrate 400 and conventional substrate 402. First, ahigh-quality thin adaptive layer 403 (i.e., corresponding to layer 22 ofFIG. 2) is grown on a conventional (first) substrate 402. The thinadaptive layer 403 needs to have a relaxed lattice constant which isvery close to that of a target material system. For example, the targetmaterial system may be Ga_(x)In_(1−x)As where, e.g., x=20%, i.e.Ga_(0.20)In_(0.80)As. In this case, Ga_(0.20)In_(0.80)As may also beselected for the material for thin adaptive layer 403, so that itsrelaxed lattice constant and crystalline lattice structure is identicalto that of the target material system for the epi layers to be grownthereon. To fabricate a thin layer 403 of Ga_(0.20)In_(0.80)As, alattice-mismatched substrate such as GaAs or InP may be used. This willgive rise to a strained-lattice layer, having a different latticeconstant than non-strained (relaxed) Ga_(0.20)In_(0.80)As.

[0049] The thickness of the thin adaptive layer 403 is preferablysmaller than its critical thickness (e.g., <100 Å) such that nodislocation generation or lattice relaxation will likely occur. Qualityand thickness control of the strained thin adaptive layer 403 ispreferable for this process. As will be understood by those skilled inthe art, after its fabrication on substrate 402, the lattice constant ofthe strained adaptive layer 403 is the same as that of the substrate 402in the direction parallel to the wafer surface. However, as describedbelow, this lattice constant of layer 403 is relaxed to its non-strainedvalue through a special process in accordance with the presentinvention.

[0050] Thus, after forming strained adaptive layer 403 on substrate 402,thin bonding layers 404 and 405 are deposited on the thin adaptive layer403 and the carrier (second) substrate 400, respectively, as shown inFIG. 4A. (Alternatively, only one of bonding layers 404, 405 may beemployed.) Then, the two substrates are put together face-to-face andbonded together, as illustrated in FIG. 4B, to form a bonding layer 406from the thin bonding layers 404 and 405. This bonding process may beaccomplished by applying heat or pressure, or a combination of both, tothe two wafers, as will be appreciated by those skilled in the art.Bonding layers 404, 405 may comprise suitable materials, such as thematerials utilized for forming interfacial bonding layer 23, or othersuitable metals or other materials.

[0051] After the two wafers are bonded together, the conventional(first) substrate 402 is etched away, leaving only the thin adaptivelayer 403 on the bonding layer 406, mounted on second, carrier substrate400 (FIG. 4C). Then, heat is applied to the carrier substrate 400, as inFIG. 4C, until the interfacial bonding layer 406 becomes liquid-like,such that the thin adaptive layer 403 can freely change its latticeconstant to its relaxed value, to relieve internal strain. After thethin adaptive layer 403 is relaxed, then the carrier substrate 400 canbe cooled down and the bonding layer 406 is re-solidified. Thus, aftercooling, this results in a now-relaxed thin adaptive layer 403 which hasa relaxed lattice constant, which is identical to, or at least closerto, that of the target material system. This layer 403 may then be usedas the alternative substrate, when mounted on a proper support, topermit epitaxial growth of the target material system, with reducedthreading dislocation defects.

[0052] In one embodiment, the result shown in FIG. 4C may be used as analternative substrate, after appropriate treating of the top (exposed)surface of thin adaptive layer 403. In this case, carrier substrate 400serves as the support. However, preferably, thin adaptive layer 403 isbonded to a new (third), bulk material support substrate 410, as shownin FIG. 4D. In this embodiment, thin adaptive layer 403 is bonded to anew bulk substrate 410, which can be a prior art semiconductor substrateor a dielectric crystal substrate that has a thermal expansioncoefficient very close to that of the thin adaptive layer 403 (to reducefracturing or damage during heating). The final step is to remove thecarrier substrate 400 and bonding layer 406. This may be done bychemical etching, or by mechanical removal by melting the bonding layer406. The exposed surface of thin adaptive layer 403, after layers 400and 406 are removed, may then be suitably treated to permit epitaxialgrowth thereon of the target material system.

[0053] Bonding layer 406 may consist of any suitable bonding layermaterial. The bonding layers 406 and 302 (FIG. 3) preferably have theproper chemical and physical properties, including: (1) no orinsignificant chemical reaction with the thin adaptive layer 403 or 303(or 332); (2) solid at room temperature, e.g., up to approximately 100°C.; (3) liquid-like form when heated to an elevated temperature (e.g.,approximately 100-500° C.); (4) physically strong hold of the carriersubstrate 400 (or the bulk material 300) and the thin adaptive layer 403(or 303) together; and (5) no or insignificant physical or chemicaldamage to the thin adaptive layer 403 (or 303) upon heating. The bondinglayers 406 and 302 can be, for example, metals (e.g., as described abovewith reference to bonding layer 23), inorganic materials, or organicmaterials.

[0054] Preferably, thin adaptive layers 303, 403 are semiconductormaterials, so that semiconductor devices may be grown thereon. Carriersubstrate 400 need not be a semiconductor, but need only be strongenough to act as a support during the process illustrated in FIG. 4.Bulk material 410, on the other hand, while it need not be asemiconductor, preferably has a thermal expansion coefficient close tothat of thin adaptive layer 403, to prevent cracking or otherundesirable effects during changes in heat. Bulk material 300 need notbe a semiconductor, but should be strong enough to support layer 302.Preferably, the thermal expansion coefficient for substrate 300 is notvery different from that of thin adaptive layer 303, although in anembodiment it need not be as closely matched as for substrate 410 andthin adaptive layer 403, because bonding layer 302 is between layer 303and substrate 300 during epitaxial growth on layer 303.

[0055] Thus, the present invention provides, in the relaxed substrateapproach, for the formation of an alternative substrate for epitaxialgrowth of a target material system. The alternative substrate comprisesa thin film adaptive crystalline layer bonded to a base substrate layer.The thin film layer may be said to have an unstrained lattice constantequal or very close to the lattice constant of the target materialsystem, where the thin film layer originally (in formation) had astrained lattice constant equal to the lattice constant of a firstsemiconductor substrate on which it is grown, which strained latticeconstant has been adjusted by bonding the thin film adaptive layer to acarrier substrate with an interfacial bonding layer and removing thefirst substrate, and then heating the interfacial bonding layer toliquidize the interfacial bonding layer to allow the strainedpseudomorphic thin film adaptive layer to relax to its unstrainedlattice structure, and then bonding the thin film layer to the basesubstrate and removing the carrier substrate to expose an epitaxialgrowth surface of the thin film adaptive layer. The thin film adaptivelayer may be referred to herein as a relaxed-strained orunstrained-strained thin film layer to indicate that it was fabricated astrained layer with a first (strained) lattice constant and thenrelaxed, via the interfacial bonding layer technique, to adjust itslattice constant to the natural, unstrained lattice constant for thematerial of the thin film layer.

[0056] As mentioned above, these alternative substrate fabricationtechniques of the disclosed embodiments, for example, for a 1.3 or 1.55μm wavelength VCSEL, also can be applied to other material systems fordifferent device applications. With reference to FIGS. 3A-3E and 4A-4E,three different exemplary material systems for the method describedabove will now be discussed.

[0057] Exemplary Material Systems

[0058] I. GaInAs/InP

[0059] The thin adaptive film 303 or 332 (or 403) can be, for example,an InGaAs thin film with an In composition approximately 15% toapproximately 45%. Its thickness can be, for example, approximately 3 nmto approximately 30 nm. The second wafer or second substrate 301 or 331(or 402) can be, for example, InP. As the lattice constant ofGa_(x)In_(1−x)As, 0.20<×<0.45, is smaller than that of InP, if a layeris grown whose thickness is less than its critical thickness, then no orlittle additional threading dislocation defects will or will likely beformed during the growth of this thin epitaxial layer. In thisembodiment, the thin film 303 or 332 (or 403) is a pseudomorphic tensilestrained epitaxial layer. The thin film 303 or 332 (or 403) can bebonded in any orientation relative to the bulk material substrate 300(or the substrate 410), which may be GaAs, InP, Si, sapphire, or othersuitable materials. Thin adaptive layer 303 or 403 may be composed ofInGaAsP, GaSb, InGaAs, InGaP, AlGaP, InSb, InP, AlSb, or InAs, SiC, Ge,GaP, InAs, GaSb, or the like. However, a relative orientation ofapproximately 0° or approximately 90° is usually preferred whenconsidering device processing control of the sample.

[0060] II. GaInAs/GaAs

[0061] The thin adaptive film 303 or 332 (or 403) can be, for example,an InGaAs thin film with an In composition between approximately 15% andapproximately 40%. Its thickness can be, for example, approximately 3 nmto approximately 50 nm. The second wafer or second substrate 301 or 402can be, for example, GaAs. As the lattice constant of Ga_(x)In_(1−x)As,where 0.15<×<0.40, is larger than that of GaAs, if a layer is grownwhose thickness is less than its critical thickness, then no or littleadditional threading dislocation defects will or will likely be formedduring the growth of this thin epitaxial layer. In this embodiment, thethin adaptive film 303 or 332 (or 403) is a pseudo-morphic compressivestrained epitaxial layer, which can be bonded in any crystallineorientation relative to the substrate 300 (or 400). The substrate 300(or 400) can be, for example, GaAs, InP, Si, sapphire, SiC, Ge, GaP,InAs, GaSb, or the like. However, a relative orientation ofapproximately 0° or approximately 90° is usually preferred whenconsidering device processing control of the sample.

[0062] III. GaSb/InAs or InAs/GaSb

[0063] The substrate 301 (or 402) can be, for example, either InAs orGaSb. Whichever material is chosen, i.e., InAs or GaSb, the thinadaptive film layer 303 or 332 (or 403) can be, for example, the otherof these two materials, and have a thickness approximately 3 nm toapproximately 30 nm. As the lattice mismatch between InAs and GaSb isless than 0.7%, the GaSb or InAs thin film is not relaxed and no orlittle threading defects should be formed in the epi layer. The thinadaptive film 303 or 332 (or 403) can be bonded in any orientationrelative to substrate 300 (or 400), which can be, for example, GaAs,InP, Si, sapphire, SiC, Ge, GaP, InAs, GaSb, or the like. However, arelative orientation of approximately 0° or approximately 90° is usuallypreferred when considering device processing control of the sample.

[0064] These are just a few examples of possible ways to fabricate CUsubstrates according to the present invention. There are many other waysto achieve a CU substrate using different III-V materials, as will beappreciated by those skilled in the art. However the principle of thepresent invention is the same as for the above examples, and these otherways are included within the scope and spirit of the present invention.

[0065] Non-limiting exemplary metal materials having low meltingtemperatures (e.g., below 600° C. or 500° C., typical temperatures usedfor MBE) that can be used as the bonding layer 302 (or 406) include, forexample, Bi, Pb, In, Sn, Sb, Al, or the like. Some alloys with lowmelting temperatures (e.g., <600° C. or 500° C.) can also be used, suchas In:Sn, Pb:Sn, In:Pb, In:Ag, and the like. The bonding layer 302 (or406) also can comprise multiple metal layers, such as layer combinationsof these metals for which some of the layers have a much higher meltingtemperature.

[0066] In summary, the present invention provides alternative substratesthat may be formed from conventional semiconductor and other bulkmaterials that facilitate growth of lattice-mismatched threadingdislocation defect-free epitaxial layers. This is accomplished throughprovision of a thin adaptive film layer, which is highly flexible due toan interfacial bonding layer that becomes liquid-like upon heating. Thepresent invention thereby facilitates the formation of a wide range ofdevices that were previously not feasible to construct due to latticemismatch constraints. A list follows of exemplary potential applicationsof this alternative substrate technology.

[0067] Exemplary Potential Applications

[0068] A variety of devices can be fabricated using the CU substrateprovided in the present invention, including electronic devices such astransistors and integrated circuits; and optoelectronics devices, suchas lasers (including diode lasers, VCSELs, and the like), LEDs andphotodetectors.

[0069] I. High-Power Mid-Infrared Lasers

[0070] Referring now to FIGS. 5A and 5B, there are shown, respectively,schematic illustrations of the epi-up configuration for laser mountingin output power measurement, according to the invention, and a prior artepi-down configuration for laser mounting in output power measurement.Pump laser 130 provides optical pumping light for the laser of epi layer125. In FIG. 5A, layer 120 may comprise layers 403 and 410 of FIG. 4, orlayers 303, 302, 300 of FIG. 3.

[0071] Sb-base type-II quantum well (QW) or superlattice (SL) lasersemitting from approximately 2 to approximately 10 μm can be grown andfabricated according to the invention on the GaAs CU substrates. Thelaser active region is composed of but not limited to eitherInAs/InGaAlSb/InAs/InAlSb type-II QWs or InAs/InGaAlSb type-II SLs. Theadvantage of growing such lasers on the GaAs CU substrate is that thelaser can be bonded on a submount 112 in the epi-down configuration, asschematically shown in FIG. 5A, instead of the prior art epi-upconfiguration, as schematically shown in FIG. 5B. This is because, inthe prior art, a substrate had to be utilized which is lattice matchedto the epi layers, which often limited the choice to materials that wereopaque to the pumping light from pump laser 130. By utilizing the CUsubstrate of the present invention, a substrate which is transparent tothe pumping light may be utilized, even though it is not lattice matchedto the epi layers 125. By using the epi-down configuration, the maximumlaser output power can be dramatically improved with better heat removalcapability from the laser active region.

[0072] II. Mid-Infrared (IR) and IR Photodetectors

[0073] High performance IR photodetectors for detecting wavelengthsapproximately 2 to approximately 25 μm can be composed ofInAlGaAs/InAlGaSb type-II SLs lattice matched to GaSb or InAssubstrates. However, both InAs and GaSb substrates highly absorbradiation at wavelengths longer than approximately 5 μm. Therefore,epi-side down mounting to the read out circuits is very difficult touse. IR photodetectors can be grown and fabricated according to theinvention on large bandgap CU substrates, and hence such photodetectorscan be integrated with read out circuits using the epi-downconfiguration to allow light to pass from the CU substrate. Analternative substrate, such as a thin GaSb layer bonded on top of a GaAsor Si substrate, can be used as a filter to filter out the visible andultraviolet (UV) spectra.

[0074] III. Visible and UV Laser Diodes

[0075] Red, orange, and yellow/green diode lasers having InGaAlPheterostructures can be grown and fabricated according to the inventionon GaAs-based CU substrates without being restricted by availablelattice-matched substrates. High-quality InGaN/AlGaN ultraviolet, blue,and green lasers with a long lifetime and low defect density can begrown and fabricated according to the invention on CU substrates. In theprior art, these devices are grown on sapphire or SiC substrates with alarge lattice mismatch. This produces a very high defect density andstrongly limits device lifetime. These devices according to the presentinvention can be used, for example, in displays, DVDs for optical datastorage, medical applications, and chemical sensors to monitorband-to-band transitions of gas species.

[0076] IV. High-Temperature, High-Power, High Voltage Electronic Devices

[0077] Transistors composed of InGaAlN and SiC heterostructures cansustain high voltage, high temperature, and deliver high power. Theseare attractive features for the power industry and the microwavecommunications industry. The electrical qualities of InGaAlN and SiCcompounds grown and fabricated according to the invention on CUsubstrates are or likely are superior to those grown on other mismatchedsubstrates for both carrier mobility and breakdown voltage.

[0078] V. High-Efficiency Visible LEDs

[0079] Red, orange, and yellow/green LEDs having InGaAIPheterostructures can be grown and fabricated according to the inventionon GaAs-based CU substrates. LEDs emitting from red to UV wavelengthscan be constructed with InGaN/InGaAlN heterostructures grown andfabricated according to the invention on Si or other CU substrates. TheCU substrates are more attractive than sapphire or SiC substratescurrently being used from both cost and electrical property standpoints.

[0080] VI. Optoelectronic Integrated Circuits and Electronic Circuitswith Mixed Materials

[0081] III-V compound lasers can be integrated with Si circuits,according to the invention, likely more easily than by using theexisting integration techniques, such as flip chip bonding and epitaxiallift off. It is also possible to work on the whole wafer instead of afraction of the wafer, as in the flip chip bonding and epitaxial liftoff techniques.

[0082] It will be understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated above in order to explain the nature of the presentinvention may be made by those skilled in the art without departing fromthe principle and scope of the present invention, as recited in thefollowing claims.

What is claimed is:
 1. A substrate comprising: a base substrate layer;and a relaxed-strained thin film adaptive crystalline layer bonded tothe base substrate layer and having a surface in-plane lattice constantdifferent from that of the base substrate layer and close to that of atarget material system.
 2. The substrate of claim 1, wherein thein-plane lattice constant is in the same range as that ofIn_(x)(Al_(y)Ga_(1−y))_(1−x)As wherein x is approximately 15% toapproximately 45%.
 3. The substrate of claim 1, wherein the substratecomprises a substrate for formation of a vertical cavity surfaceemitting laser based on In_(x)(Al_(y)Ga_(1−y))_(1−x)As.
 4. The substrateof claim 3, wherein x is approximately 15% to approximately 45%.
 5. Thesubstrate of claim 1, wherein the thin film adaptive crystalline layercomprises InGaAs having an In composition between approximately 15% andapproximately 45%.
 6. The substrate of claim 1, wherein the basesubstrate comprises GaAs, and the thin film adaptive crystalline layercomprises In_(x)(Al_(y)Ga_(1−y))_(1−x)As.
 7. The substrate of claim 6,wherein x is approximately 15% to approximately 45%.
 8. The substrate ofclaim 1, wherein the thin film adaptive crystalline layer comprises asemiconductor.
 9. The substrate of claim 8, wherein the semiconductorcomprises InGaAsP, GaSb, InGaAs, InGaP, AlGaP, InSb, InP, AlSb, or InAs.10. The substrate of claim 1, wherein the base substrate layer comprisessemiconductor, an inorganic material, a metal, or a combination thereof.11. The substrate of claim 10, wherein the semiconductor comprises GaAs,InP, GaP, Si, or Ge.
 12. The substrate of claim 10, wherein theinorganic material comprises sapphire, poly-crystalline boron nitride,or ceramics.
 13. The substrate of claim 10, wherein the relaxed-strainedthin film adaptive crystalline layer is fabricated having a strainedlattice constant equal to a lattice constant of a first semiconductorsubstrate on which it is grown, which strained lattice constant has beenadjusted by bonding the thin film adaptive layer to a carrier substratevia an interfacial bonding layer and removing the first substrate, andthen heating the interfacial bonding layer to liquidize the interfacialbonding layer to allow the strained thin film adaptive layer to relax toits unstrained lattice structure to form the relaxed-strained thin filmadaptive layer, and then bonding the thin film layer to the basesubstrate and removing the carrier substrate to expose an epitaxialgrowth surface of the thin film adaptive layer.
 14. A method of forminga substrate for formation of semiconductor devices, comprising: forminga strained pseudomorphic thin film adaptive layer on a first substrate;bonding a first surface of the thin film adaptive layer to a carriersubstrate with an interfacial bonding layer; removing the firstsubstrate by selective etching or lift off and leaving the thin filmadaptive layer; and liquidizing the interfacial bonding layer to allowthe strained pseudomorphic thin film adaptive layer to relax itsunstrained lattice structure.
 15. The method of claim 14, furthercomprising the steps of: bonding the surface of the thin film adaptivelayer to a second substrate; and removing the carrier substrate toexpose a second surface of the thin film adaptive layer.
 16. The methodof claim 15, further comprising treating the surface of the secondsubstrate prior to the bonding.
 17. The method of claim 14, wherein theliquidizing comprises heating the interfacial bonding layer.
 18. Asubstrate produced in accordance with the method of claim
 14. 19. Anoptoelectronic apparatus, comprising: a substrate comprising: a thinfilm adaptive crystalline layer; and a base substrate layer, the thinfilm adaptive crystalline layer bonded to the base substrate layer andhaving a surface in-plane lattice constant, and wherein the in-planelattice constant is different from that of the base substrate layer,wherein the thin film adaptive crystalline layer comprises a strainedpseudomorphic thin film grown on a first semiconductor substrate that isbonded to and transferred to a second carrier substrate, wherein thefirst semiconductor substrate is removed and the in-plane latticeconstant relaxes from an original strained value to a new value close toan unstrained lattice constant, and wherein the thin film adaptivecrystalline layer is physically or chemically bonded to the basesubstrate layer with or without an interfacial bonding layer; and anoptoelectronic device epitaxially grown on the thin film adaptivecrystalline layer.
 20. The optoelectronic apparatus of claim 19, whereinthe optoelectronic device is a semiconductor laser.